The invention relates generally to marine engines, and more particularly, to propeller hubs.
Outboard engines include a drive shaft which extends from the engine power head, through an exhaust case, and into an engine lower unit. The lower unit includes a gear case, and a propeller shaft extends through the gear case. Forward and reverse gears couple the propeller shaft to the drive shaft. The drive shaft, gears, and propeller shaft sometimes are referred to as a drive train.
A propeller is secured to and rotates with the propeller shaft. Torque from the propeller is transmitted to the shaft. Specifically, propeller hub assemblies transmit torque to the propeller shaft. Exemplary propeller hub assemblies include cross bolts, keys, shear pins, plastic hubs, and compressed rubber hubs. Such hub assemblies should have sufficient strength or stiffness so that during normal engine operations, very few losses occur between the propeller shaft and the propeller. Such hub assemblies, however, also should be resilient so that the engine drive train is protected in the event of an impact, e.g., if the propeller hits a log or rock.
These and other objects may be attained by a propeller assembly that includes, in an exemplary embodiment, a spiral wound spring for securing a propeller to a propeller shaft. In the exemplary embodiment, the propeller assembly includes a thrust bearing which tightly fits to an inclined surface of the propeller shaft. The thrust bearing includes a hub mating surface which forms a tight fit with one end of a hub of the propeller.
The propeller hub has a cylindrical shape, and a plurality of blades extend from an outer surface of the hub. A bore extends through the propeller hub, and the spring fits securely within the hub bore. The spring is secured, e.g., welded, at one end to an interior surface of the hub. The assembly further includes a washer and a nut which engages to a threaded end of the propeller shaft.
To secure the propeller assembly to the propeller shaft, the thrust bearing is pushed over the propeller shaft and into tight fit with the shaft inclined surface. The propeller hub is pushed over the propeller shaft until the one end of the hub is in tight fit with the hub mating surface of the thrust bearing. The propeller shaft extends through the spring, and an end of the spring opposite the spring end secured to the hub inner surface forms a tight fit with the propeller shaft. The washer and nut are then pushed over the propeller shaft and the nut is secured to the shaft so that the thrust bearing, hub, and washer are tightly secured to the propeller shaft.
During operation, the torque from the propeller shaft is transmitted through the spring to the propeller hub. The spring is stiff enough so that under normal operation, the propeller, spring, and propeller shaft rotate together. The tight fit between the thrust bearing, the propeller shaft, and propeller hub also facilitates transmission of torque from the shaft to the propeller hub. Upon the occurrence of an impact, the propeller shaft may rotate relative to the propeller hub. Under such operating conditions, the spring axially twists as the propeller shaft rotates relative to the hub. That is, the end of the spring secured to the hub rotates with the hub, and the end of the spring secured to the shaft rotates with the shaft. Relative rotation between the shaft and the hub results in axial twisting of the spring.
In the event that the impact is sufficient to loosen the initial tight fit between the thrust bearing and the hub, then the propeller shaft may continue to rotate relative to the hub until spring is completely rotated to its maximum rotation. If the forces are not sufficient to cause the spring to break, the spring then causes the propeller to once again rotate with the shaft in a limp home mode.